I have a lot of file core wrrote every 6 hour and the same time of alert"node".log file show error about PRVF-5424 : Clock time offset check failed.
Yesterday I use this:

[root@hostname2 ~]# service ntpd stop
[root@hostname2 ~]# ntpdate
[root@hostname2 ~]# service ntpd start
in order to resolve but today the problem re-begin?

2013-04-17 20:23:06.063
CRS-10051:CVU found following errors with Clusterware setup : PRVF-5424 : Clock time offset check failed
PRVF-5413 : Node "" has a time offset of 69695.6 that is beyond permissible limit of 1000.0 from NTP Time Server "******"
PRVF-5413 : Node "" has a time offset of 69940.6 that is beyond permissible limit of 1000.0 from NTP Time Server "**

2013-04-18 02:23:08.973
CRS-10051:CVU found following errors with Clusterware setup : PRVF-5424 : Clock time offset check failed
PRVF-5413 : Node "" has a time offset of -1826.9 that is beyond permissible limit of 1000.0 from NTP Time Server "1***
PRVF-5413 : Node "" has a time offset of -1364.2 that is beyond permissible limit of 1000.0 from NTP Time Server "1**

2013-04-18 08:23:16.093
CRS-10051:CVU found following errors with Clusterware setup : PRVF-5424 : Clock time offset check failed
PRVF-5413 : Node "" has a time offset of -2698.5 that is beyond permissible limit of 1000.0 from NTP Time Server "1***
PRVF-5413 : Node "" has a time offset of -1998.7 that is beyond permissible limit of 1000.0 from NTP Time Server "1***

In general you can solve these issues by configuring NTP service running on your cluster nodes to be synchronized with other NTP servers. For example on Linux please read "Oracle RAC and NTP" in http://www.oracle-base.com/articles/linux/linux-ntp-configuration.php#oracle-rac-and-ntp.